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  On Sunday 15 Oct 1995 at the Historical Diving Society stand at the diving
exhibition in Birmingham (UK) I saw a video describing old diving gear.
  (1) It mentioned the Rouquayrol-Denayrouze 1860's aqualung-like backpack air
supply for a suit-and-helmet, with a 30-bar backpack air sphere, and a genuine
demand valve that long ago. As regards why the design didn't survive until
proper gas cylinders could be made: reading `20000 Leagues Under the Sea'
shows that Jules Verne had no conception of a demand value; but the video
showed an illustration from an early edition of `20000 Leagues Under the Sea'
that showed the Nautilus's divers with correctly drawn Rouquayrol-Denayrouze
sets and hardhat-type suits but no airlines to the Nautilus or the surface. In
the book Jules Verne wildly exaggerated the dive duration of those sets to 6
to 8 hours. But he <did> know about the habit of divers being unwilling to
come out of the water into rain so they wouldn't get wet.
  (2) It mentioned the Commeinhes French aqualung invented in 1937 and
accepted by the French Navy. It was as depicted below. The box between the
cylinders is an enormous demand regulator.
  ________
 (________)
 |________|
 (________)
  (3) It described Ohgushi's `Peerless Respirators' constant-flow air set
early this century. The Japanese Navy used it for practical diving. It had 2
cylinders. Their valves were at the bottom so the diver could reach them. It
was also advertized as an industrial breathing set on land.
  (4) It described Le Prieur's French constant volume sets invented for sport
diving in the 1930's in the south of France. The video showed the cylinder
worn valve forwards on the front of the abdomen with the rear of the cylinder
projecting parallel to the legs like the thigh of a 3rd leg. Likely that
position was to avoid skew drag caused by the other position that I have seen
Le Prieur set cylinders in, which was diagonally across the chest and belly.
All that to get the cylinder valve within easy reach of the diver while
swimming.

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